Pressurized water nuclear reactors include a vessel of generally a cylindrical shape, containing the core of the nuclear reactor and arranged with its axis vertical in a cylindrical reactor pit having a bottom end located in line with the vessel. The core of the nuclear reactor is cooled by pressurized water flowing, in contact with the fuel assemblies, in the primary circuit of the reactor and inside the vessel.
In the event of certain accidents occurring in the reactor and leading to loss of operation of the core cooling, consideration must be given, in view of the very grave potential consequences, even though the probability of such an event is extremely small and practically zero, to the case in which the safety injectors of the reactor might not enter into operation. An accident sequence may then occur which leads to meltdown of the core and of the internals of the reactor in the absence of cooling water, which can cause destruction of the vessel bottom head by perforation, and flow of the core mass and of the materials surrounding the core into the concrete pit containing the reactor vessel.
Contact of the molten fuel mass and of the materials surrounding the fuel, called corium, the temperature of which may reach values of the order of 2500.degree. C. in the absence of cooling, may cause complete destruction of the reactor pit bottom.
During normal operation of the reactor, without an accident, the ambient conditions in the reactor pit are very severe. In fact, permanent irradiation takes place, which increases over time during the lifetime of the reactor. A cumulative dose rate over the predicted 40 year lifetime of a reactor may reach 280 Mrad.
It is important, in the scenario of such a coolant loss accident, to monitor the development of the phenomenon and, in particular, the development of the condition of the vessel bottom head, in order to determine whether the latter is partially or completely melted, and whether the corium is flowing through the vessel at only a few points or over the entire surface of the bottom head.
The ambient conditions in the reactor pit, in the event of perforation of the vessel, make it difficult to place cameras in the pit in order to display the development of the phenomenon on a screen. In fact, the vapors of the molten materials, added to the cooling steam, will cause blinding of the monitoring device.
To date no efficient method for detecting perforation of the vessel bottom head of a nuclear reactor and for monitoring the development of the condition of the vessel bottom head in the event of core meltdown has been known.